This invention generally relates to mechanisms for transferring force through a pulley and belt, or a chain and sprocket, and more particularly to such mechanisms, for example in the pedaling arrangements of bicycles or the like, wherein drive ratios are changed by moving the belt or chain between pulleys or sprockets of different diameters during rotation of the pulleys or sprockets.
The advantages of and need for variable speed pulley and belt, or chain and sprocket assemblies has long been recognized. U.S. Pat. Nos. 3,448,628 and 3,583,249 show deraileur mechanisms of the type commonly found on bicycles for the purpose of shifting a driving chain of a pedaling system from sprocket to sprocket in a cluster of sprockets having different diameters, for example. Other devices, such as those shown in U.S. Pat. Nos. 3,798,989, 3,861,277 and 3,800,613 show mechanisms wherein the effective diameter of a compound sprocket is varied to achieve different speed ratios.
While these previously known devices may be satisfactory for many purposes, it is believed that they may be improved upon in several respects. For example, a derailuer mechanism does not always shift a chain precisely, and this frequently results in over or under shifting and a need to adjust the shift levers after a shift has nominally been completed. Also, to shift a chain with a deraileur mechanism, the chain is pushed off one sprocket and onto another, which can cause a momentary loss of power to the rear wheel of the bicycle, and can cause the chain to slip when the chain tension is high, such as when the bicycle is being ridden up a hill.
U.S. Pat. No. 4,127,038 discloses a sprocket cluster that is pivoted to shift a chain from one sprocket to another while maintaining a positive drive relationship between the chain and the sprockets. As a practical matter, this pivoting movement tends to place a relatively low limit on the number of sprockets that the cluster can include.